For such a Love It or Hate It series, Guilty Crown seems, at times, hell-bent on challenging two certain othershows for the title of most Whams in a single series. Up until episode 12, it seemed like just another awesome action show, but after Shu kills Gai in the aforementioned episode, it becomes a whole lot Darker and Edgier. To the point that it's easier to count the episodes that don't qualify.
Episode 5: Inori reveals to Shu that she basically Honey Trapped him to get him to join Funeral Parlor by Gai's orders, and the episode ends with Gai's possible death after the GHQ attacks his location with their Leukocyte.
Episode 5 leaves Gai's fate unknown as the viewers are treated to this◊ view of what Leukocyte can do. And it's not even fully charged at that point. It's firing at 3%!
Episode 8: Shu and Funeral Parlor manage to infiltrate a facility which contained the very source of the Apocalypse Virus, only to see that the canister which contained it was already taken by Shuichiro of the GHQ.
Episode 8 is where the endless chain of Guilty Crown Wham begins when the meteor rock that started Lost Christmas is stolen by Keido. And this was a Beach Episode!
Episode 9: Yahiro, who has been absent for the last few days, reveals to Shu that he and his brother Jun have been on the run from the GHQ because they wanted to euthanize Jun. Later, when they are found by the GHQ, Jun reveals to Shu his past memories with his brother before the Lost Christmas incident, and Shu is forced to Mercy Kill him afterwards. When Yahiro regains consciousness and asks for his brother, a traumatized Shu reveals to him the deed, unaware that they were both watched by Hare.
Episode 9 gave us the first of many, manyHeroicBSODs Shu would have to face. After Shu meets Yahiro's younger brother Jun, who's suffering from an advanced stage of the Apocalypse Virus, it's explained how the virus gave Jun the power to see people's Voids, allowing him to see the darker sides of people, including his older brother Yahiro. Jun begs Shu to kill him so he can die while he still remembers Yahiro's kinder side, not the side with a killing Void. As the virus drives Jun insane, he manages to seize control of an Endlave and nearly crushes Yahiro to death with it. Shu is forced to Mercy Kill Jun. The episode ends as Shu admits what happend to Yahiro, still in shock from he just did.
Episode 10: Shu has quit Funeral Parlor and is in the throughs of a Heroic BSOD from the events of the last episode. Keido and his allies in GHQ have used the metorite that started Lost Christmas to trigger another even revealing himself to be the one who triggered the first one and (probably) the show's Big Bad. Gai and most of Funeral Parlor appear to have been killed and Inori appears to be in agony because of the incident.
Episode 11: Inori reverses the effects of the virus signal by singing. Shu and the club is coming to rescue Funeral Parlor. Everything looks good until the last five minutes, when Keido's blond accomplice teleports in, reveals he has the Void Genome by extracting Inori's sword and nearly killing Shu. Gai takes the hit while he escapes with Inori, causing the signal to rebroadcast and hell to come roaring back. Here we go again.
Episode 12 gives us the true cause of Lost Christmas and the nature of the girl that strangely ressembles Inori, Mana. The girl was the first person to be infected by the virus and Shu's older sister. It's also confirmed that Gai and Shu were friends as children, that Shu was the brave one in the bunch, that the virus originally had marginal crystallization effects and more so affected the victims behavior, making them unstable and sociopathic. This in turn made Mana fall head over heels for her little brother (with some heavy Yandere tendencies for added squick, tricked young Gai into shooting himself, and propose to and subsequently scare the ever loving crap out of Shu. He calls her a monster, so Mana freaks out and subconsciously mutates the Apocolypse Virus into what the viewers are familiar with and rapidly spreads it, at which point Lost Christmas ensues. Wow.
In Episode 13, Keido somehow becomes the new President of Japan, and his first order is to completely seal off the "infected" portion of the city, including Shu and his school.
In Episode 14, Keido plans on eliminating said area entirely, beginning by shrinking the boundaries, destroying buildings and killing people in the way. Meanwhile, Shu becomes king of his school, ranking students by their Void power. This could end poorly.
It DOES end poorly, as shown in Episode 15. Souta and some others plan to break into the hospital to get more of the vaccine to prove that their Voids are useful, and Shu and Hare pursue them. In the end, they're all spotted by the Anti Bodies, and Hare is killed, essentially because of Souta's group. This results in Shu going berserk, Shinji-style, and in the end going against his own principles of not wanting a Void Ranking System, a system to determine who's more important to keep alive. Can it possibly get any worse now? Oh, and he also forcibly extracts Inori's Void, which when taken out of context, can be interpreted as statutory rape. If they keep this up, there's no telling where the show is going to wind up.
In Episode 16, we see just what kind of kingdom Shu makes, where he forces Rank F students undergoing Stage 1 to work for their vaccine, and how much of a Jerkass he becomes. At the end of the episode, he has a Heroic BSOD when he figures out that breaking a person's Void causes them to instantly succumb to the virus, and that was how Hare died. Then, he pulls out Argo's Void and laughs while considering breaking it and killing Argo. Then after the credits, we see that Shu's mother has brought Gai back to life.
And then there's the fact that, as of Episode 16, Mana, Shu's reality-warping and incestuous elder sister has effectively stolen Inori's body, starting while in the garden with him. Or when she killed those students in Episode 14.
In Episode 17, Arisa leads a coup d'etat, Back from the Dead Gai cuts off Shu's right arm and takes the Void Genome, and the UN vote to eliminate Japan through an air-strike. And it wouldn't be a surprise if they actually choose to just carpet-nuke all of Japan, to eliminate the Apocalypse Virus' presence there.
Episode 18: Gai sacrifices a bunch of Mauve Shirt students to repel the UN attack, GHQ activates a massive Kill Sat network to threaten the world not to intervene in their plans, Daryl has a Heel-Face Turn, Arisa kills her own grandfather, and Inori gets captured after leading the GHQ away from Shu.
Episode 19: Shuichiro reveals himself to be Haruka's older brother and the one who murdered Shu's father in the past. Shu uses up the last remaining Void Genome given to him by Haruka to destroy Segai's forces and kill Segai himself. And to everyone's shock, not only did Shu absorb Souta's Void, but also his Apocalypse infection as well.
Episode 21: At the end of the penultimate episode, Mana is finally revived.